An active matrix organic light emitting diode (hereinafter, referred to as ‘AMOLED’) display device is a spontaneous emission unit configured to illuminate an organic light emitting layer by recombination of electrons and holes, which has high luminance and low driving voltage, and is capable of being ultra-thin, and thus, is expected to be a next generation display device.
Each of the plurality of pixels forming the AMOLED display device includes a light emitting unit having an organic light emitting layer interposed between an anode and a cathode and a pixel circuit configured to independently drive the light emitting unit. The pixel circuit is classified into a voltage driving compensation circuit and a current driving compensation circuit. The voltage driving compensation circuit is a type for applying data voltage to the pixel circuit, and the current driving compensation circuit is a type for applying data current to the pixel circuit. The voltage driving compensation circuit and the current driving compensation circuit have commonality in storing data voltage in a storage capacitor connected to a gate of a driving unit as a result of operation processes thereof.
Meanwhile, in order to apply data voltage to each of the pixels in the voltage driving compensation circuit, first, a parasitic capacitor of a line is required to be charged and discharged. The voltage driving type is easier to charge/discharge than the current driving type, and thus, has a fast pixel operating speed, and is easy to connect with signals of a display driving circuit. All of the driving voltage pixel compensation circuits have a period of self-compensating a critical voltage of the driving unit. In a conventional critical voltage compensation method, the critical voltage of the driving unit is detected and charged in the storage capacitor, and is offset when an OLED current flows, and thus, an effect thereof is removed. However, since a difference of electron mobility generated in a process of switching thin film transistor (hereinafter, referred to as ‘TFT’) units is not stored or compensated by the circuit, the difference of the electron mobility generated in the process of the TFT unit is not theoretically compensated. Also, in the voltage driving compensation circuit, additional signal lines and TFT units configured to compensate for a change of the critical voltage share a large space of an entire pixel area, and thus, the opening ratio is greatly decreased.
The current driving compensation circuit is advantageous in receiving a current from a data driving IC and storing the current in a scan period, and then, the current flows in an OLED light emitting period. The current driving compensation circuit is advantageous in compensating for mobility as well as the difference of the critical voltage. Also, since the current driving compensation circuit is not affected by a voltage drop phenomenon of a supplied voltage, the current driving compensation circuit has a structure for ideally and stably supplying an OLED current. However, since the storage capacitor in the circuit is required to be charged by the data current, a charging time requires long time in a low data current level by the parasitic capacitor portion of the data line, and a long time is required to drive each pixel. In particular, the above property has a problem of increasing a time for charging a pixel in a high resolution and large sized panel. In order to solve the above problem, a pixel circuit using a current mirror structure is developed and a pixel charging time is minimized, but an error is generated when electric characteristics of a mirror unit are different from that of the driving unit. Also, since currently commercialized driving ICs use the voltage driving type, an additional cost is required to manufacture an additional driving IC.
Meanwhile, among element unit technologies of the pixel circuit included in the display, an amorphous silicon TFT has characteristics of uniformly maintaining electron mobility even in a large sized substrate and in established manufacturing technology, and is first considered in the development of a large sized AMOLED display technology. However, the amorphous silicon TFT has poor characteristics in electric stability due to unique characteristics of an amorphous silicon layer. The most important problem caused by the unstability of the amorphous silicon TFT is a change of the critical voltage caused by a stress from a continuous gate bias.